SAN DIEGO (AP) - Interstate 8 has long been a political divide in San Diego and maybe never more than in the campaign to replace Mayor Bob Filner, who resigned amid a torrent of sexual harassment allegations.

Republican Kevin Faulconer, a city councilman, easily topped a field of 11 candidates in a first round of voting by dominating in newer, wealthier neighborhoods north of the freeway. Democrat David Alvarez, also a councilman, finished second by cleaning up south of the freeway in more densely populated, predominantly Latino areas.

If Faulconer prevails in Tuesday’s runoff, San Diego would be the nation’s largest city with a Republican mayor. He would be the only Republican to lead a major city in California, where Democrats hold all statewide offices.

If Alvarez wins, he would be the first Latino mayor in the city’s 164-year history. To overcome the 18-point gap between him and Faulconer in the first round, he must have a strong showing south of Interstate 8 among voters who made Filner the city’s first Democratic leader in 20 years in 2012.

Tom Shepard, a longtime strategist who ran campaigns for both parties, said the north-south divide has shaped San Diego politics for as long as he can remember. Republicans solidified their base north of the freeway as bedroom communities sprang up in recent decades.

“The only difference over the last 10 years is that voters south of the 8 are very informed, organized labor is turning them out and they’re capitalizing on their numbers better than they used to,” said Shepard, who is not involved in the race.

Latinos make up 29 percent of the city’s 1.3 million residents. The San Diego County registrar of voters says they account for 18 percent of the electorate.

The National University System Institute for Policy Research says 60 percent of votes in the first round were cast north of Interstate 8, and turnout there was significantly higher than it was south of the freeway. The registrar estimates Tuesday’s overall turnout around 45 percent.

Mail-in ballots are expected to favor Faulconer, so Election Day turnout is crucial for Alvarez.